Theresa May has said she intends to trigger Article 50 by the end of March and the EU's chief negotiator said on Tuesday that would give a target of October 2018 for the Brexit deal to be agreed.

The government is appealing against a High Court ruling last month that it could not invoke Article 50 of the Lisbon Treaty, which begins a two-year exit process from the European Union, without parliamentary assent. (Source: File)

The British government will need to put forward a parliamentary bill to trigger formal European Union divorce talks should it lose a Supreme Court fight over who can start the Brexit process, a government lawyer said on Tuesday. However, this could be just a one-line piece of legislation, James Eadie added — an option which could limit anti-Brexit lawmakers’ ability to delay the process or water down government policies on the terms of the EU exit. “It would require not just parliamentary involvement … but primary legislation,” Eadie told the Supreme Court.

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“The reason it requires primary legislation is (because) you are being asked to declare … unlawful the exercise of the prerogative power to give Article 50 notice as the first step in the process,” he told the 11 justices. He was referring to an executive power known as “prerogative” which, under Britain’s unwritten constitution, allows the government to take certain actions without going through parliament.

“If the Supreme Court decides against our arguments here then the solution in legal terms is a one-line act,” Eadie said on day two of a four-day hearing. “Maybe that would lead to all sorts of parliamentary complication and possible additions and amendments and so on, but that’s the solution.”

The government is appealing against a High Court ruling last month that it could not invoke Article 50 of the Lisbon Treaty, which begins a two-year exit process from the European Union, without parliamentary assent. Prime Minister Theresa May has said she intends to trigger Article 50 by the end of March and the EU’s chief negotiator said on Tuesday that would give a target of October 2018 for the Brexit deal to be agreed.

Eadie said by giving approval for June’s referendum on whether Britain should leave the EU, parliament had accepted that Article 50 would have to be triggered in the event of a “leave” vote as that was the only way to implement the result. Britons voted for Brexit by 52 to 48 percent and the government has said this mandated it to begin the divorce process using the prerogative power.

British judges are not supposed to compel parliament to legislate and Eadie said this was what the Supreme Court was being asked to do. Noting that there had already been several debates about Brexit in parliament, including one called by the opposition Labour Party, Eadie said lawmakers had not called for primary legislation to trigger Article 50 during those debates.

“Parliament does not seem to want the obligation that the (High Court) has thrust upon them,” he said. There had been speculation that in the wake of a Supreme Court defeat May might seek to get lawmakers’ approval by a parliamentary motion which could be brought forward quickly and avoid the detailed scrutiny that a new bill might elicit.

Investors believe that greater parliamentary involvement would reduce the chances of a “hard Brexit” in which tight controls on immigration are prioritised over European single market access. May is facing a rebellion from her own lawmakers on Wednesday when parliament debates whether the government should set out its Brexit plan before triggering Article 50.

The pound hit a two-month high against the dollar on Tuesday as investors bet the government would lose the Supreme Court case. The verdict is expected in January.